While a number of applications exist for ammonium paratungstate (APT) powders, a particularly important application is their use as a source material in the production of tungsten and tungsten carbide powders. In this regard, it is desirable to produce ammonium paratungstate powders having specific particle distributions to influence the size of the resultant metal and carbide powders. It is particularly advantageous to be able to produce small size APT powders (mean particle size less than 80 microns) for use in producing small metal and carbide grades (mean particle size between 0.5 to 2 microns).
APT can be made from a number of processes. The primary hydrometallurgical synthesis used involves the formation of a sodium tungstate solution. This solution can then be purified and converted into an ammonium tungstate solution which is then evaporated to yield ammonium paratungstate. The particle size of crystalline APT powders precipitated from solution can be reduced by milling the powder. However, the milling introduces unwanted contamination into the APT powder. Smaller seed crystals of APT can be added to the saturated solutions to induce the formation of smaller APT crystals but milling APT to produce the desired size for the seed crystals may still lead to contamination of the precipitated crystals. Thus, it would be desirable to be able to produce APT powders having the desired particle sizes without milling.